Information can be displayed on electronically modulated surfaces such as liquid crystal displays (LCDs). Such displays can be used for signage, shelf labels, or large scale displays such as billboards.
Various types of LCDs are known in the art. Flat panel LCDs can use two transparent glass plates as substrates, as described in U.S. Pat. No. 5,503,952. Such displays are expensive and bulky. Flexible, electronically-written display sheets using nematic liquid crystals materials are disclosed in U.S. Pat. No. 4,435,047. The sheets can be thin glass, or a polymer, for example, Mylar polyester. The nematic liquid crystals require continuous electrical drive to remain transparent. U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a chiral nematic liquid crystal (cholesteric liquid crystal, or ChLC) in polymeric domains contained by patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light, and a light scattering focal conic state. These two states are stable and can be maintained in the absence of an electric field. This enables larger displays.
Various drive schemes are known for use with liquid crystal displays. For example, U.S. Pat. Nos. 5,251,048 and 5,644,330 disclose driving methods to switch chiral nematic materials between stable states. However, the update rate of these displays is about 10-40 milliseconds per line of the display, which is too slow for most practical applications. For example, it would take 10-40 seconds to update a 1000 line display. U.S. Pat. Nos. 5,748,277 and 6,154,190 disclose fast driving schemes, called dynamic driving schemes, for chiral nematic displays. The dynamic driving schemes described include a preparation step 1, selection step 2, and evolution step 3, as shown in FIG. 1A, and optionally further include a pre-holding step 4 and a post-holding step 5 as shown in FIG. 1B. The driving schemes require complicated electronic driving circuitry. For example, all column and row drivers for a display must output bi-polar and multiple level voltages. The driving scheme results in the appearance of an undesirable black bar shifting across the display during image writing. U.S. Pat. No. 6,268,840 discloses a unipolar waveform drive method to implement the above-mentioned dynamic driving schemes. However, because the preparation step, the selection step, and the evolution step each require distinct voltage amplitudes, both column and row drivers are required to generate multilevel unipolar voltages.
Rybalochka et al. describes U/√2 dynamic driving schemes in Simple Drive Scheme for Bistable Cholesteric LCDs, SID 2001, pp. 882-885, and in Dynamic Drive Scheme for Fast Addressing of Cholesteric Displays, SID 2000, pp. 818-821. The U/√2 dynamic driving scheme requires a two-level column driver and a two-level row driver, which output either U or 0 voltage, as shown in FIGS. 1C and 1D. These drive schemes for producing focal conic or planar states do not produce undesirable black shifting bars during writing, but cause the entire frame to go black during the writing.
U.S. Patent Application Publication No. 2002/0109661 A1 discloses a drive scheme for a gray scale bistable cholesteric reflective display utilizing variable frequency pulses. The addressing method includes applying a predetermined number of pulses to a first plurality of electrodes, and applying a like number of the predetermined number of pulses to a second plurality of electrodes. Each of the predetermined number of pulses has a different frequency, wherein the predetermined number of pulses are applied within a set time period. This disclosure utilizes multiple voltage sources as well as multilevel display drivers, which adds cost and complexity to the power supply and display drivers.
U.S. Patent Application Publication No. 2003/0085863 A1 discloses a dynamic drive scheme wherein multiple voltages are used to supply a pulse to the liquid crystal between the transient planar state and the stable planar state to drive the display to the focal conic state. More than two voltages are used to derive the appropriate waveforms for the drive scheme. The use of the drive scheme as applied to gray scale displays is disclosed.
There is a need for a simple, low cost, and fast drive scheme for cholesteric liquid crystal displays that is capable of achieving a gray scale of multiple gray levels using a two-level voltage driving method.